Searching for Quirks at LHCb

This paper proposes a novel search strategy using the LHCb Vertex Locator's unique forward geometry and software trigger to detect the distinctive back-to-back, planar hit patterns of quirk pairs, thereby probing parameter regions inaccessible to current ATLAS and CMS searches.

The Gist

Imagine the Large Hadron Collider (LHC) as a massive, high-speed particle racetrack. Scientists usually look for new particles by watching what happens when two particles smash together and fly apart in all directions. But there's a specific type of hypothetical particle called a "Quirk" that is very hard to catch because it doesn't play by the usual rules.

Here is a simple explanation of what this paper proposes, using everyday analogies.

The Mystery of the "Quirk"

Think of a Quirk and its partner (an anti-Quirk) as a pair of dancers who are tied together by an invisible, super-strong rubber band.

• The Rubber Band: This isn't a normal rubber band; it's a "flux tube" created by a hidden force.
• The Dance: When they are created in a collision, they try to fly apart. But as they separate, the rubber band stretches. Eventually, the tension gets so high that the band snaps them back together.
• The Result: Instead of flying away in a straight line like normal particles, they oscillate back and forth, crossing each other's paths multiple times. It's like two people running in a figure-eight pattern, tied together, while a strong wind (the magnetic field of the detector) tries to push them sideways.

The Problem: Why We Haven't Found Them Yet

The big detectors at the LHC (ATLAS and CMS) are like giant, round stadiums surrounding the collision point. They are great at catching things that fly outward in all directions.

• The Issue: Because Quirks are tied together, they don't fly outward very far. They mostly stay close to the center of the track, bouncing back and forth.
• The Missed Opportunity: The current detectors often require particles to fly fast and far to trigger an alarm. Since Quirks stay close and move in a weird, looping pattern, the alarms often don't go off, or the detectors miss the complex path they take.

The New Idea: The "Side-Angle" View

The authors of this paper suggest using a different detector called LHCb, specifically a part of it called the VELO (Vertex Locator).

• The Analogy: If ATLAS and CMS are like cameras taking a photo of the whole stadium, LHCb is like a high-speed camera placed right next to the starting line, looking down the length of the track.
• Why it helps: Because Quirks mostly move forward or backward along the track (rather than flying sideways), they spend a lot of time right in front of the LHCb camera.
• The "Back-to-Back" Pattern: The VELO is made of many thin layers of sensors. As the Quirk pair bounces back and forth, they will leave a very specific pattern of "footprints" (hits) on these sensors. They will hit sensors on opposite sides of the track at the same time, creating a perfect, flat, back-to-back pattern.

The Plan: How to Catch Them

The paper proposes a new way to search for these particles using the LHCb detector:

1. The Trigger: The LHCb detector has a smart, software-based system that can look at every collision in real-time. The authors suggest programming this system to look specifically for that weird "back-to-back" pattern of hits, rather than just looking for things flying fast.
2. The Filter: They plan to use simple geometry rules: "Did we see two hits on opposite sides of the track? Are they in a straight line? Did this happen in several layers in a row?"
3. The Background Check: They checked if normal particles (like photons turning into electron-positron pairs) could fake this signal. They found that while a single fake pair might happen, it is extremely unlikely for normal particles to create a long, consistent chain of back-to-back hits across multiple layers.

What They Found

Using computer simulations, the authors showed that:

• LHCb can see what others can't: There is a "blind spot" in the current search results where Quirks could be hiding (specifically where the rubber band tension is just right). LHCb is uniquely positioned to look in this blind spot.
• High Sensitivity: Even with a relatively small amount of data (what they expect to collect in 2026), LHCb could either find these particles or rule out a huge range of possibilities that other experiments haven't been able to check.

The Bottom Line

This paper is a proposal to change the "search strategy." Instead of looking for particles flying outward in a stadium, they want to look down the hallway of the LHCb detector for a pair of particles tied together by an invisible string, bouncing back and forth. If they exist, the unique geometry of the LHCb detector makes it the best place in the world to find them.

Technical Summary

Technical Summary: Searching for Quirks at LHCb

Problem and Motivation
Quirks are hypothetical heavy particles charged under both the Standard Model (SM) and a new hidden confining force. When a quirk-anti-quirk pair is produced, a flux tube (string) forms between them. Unlike QCD, where light quarks are pulled from the vacuum to form mesons, the flux tube in the quirk scenario stretches until the potential energy pulls the pair back together. This dynamic results in macroscopic oscillations, with typical separations ranging from centimeters to tens of meters depending on the quirk mass ($m_Q$) and the confinement scale ($\Lambda$).

Existing experimental constraints on quirks, primarily from ATLAS and CMS, rely on searches for Heavy Stable Charged Particles (HSCPs) or monojet signatures. However, these central detector searches face limitations: HSCP searches are effective only when the string force is negligible (small $\Lambda$), while monojet searches require significant initial-state radiation (ISR) to trigger, suppressing signal rates for low-recoil systems. Consequently, a significant region of the parameter space, particularly at intermediate $\Lambda$ values ($\sim 1000$ eV), remains weakly constrained. The authors propose that the LHCb experiment, with its forward geometry and unique detector capabilities, offers a complementary and potentially superior avenue for discovery in this unexplored regime.

Methodology
The proposed analysis leverages the LHCb Vertex Locator (VELO), a high-precision silicon pixel detector located close to the interaction point. The methodology relies on the distinct kinematic and geometric signatures of quirk pairs:

1. Forward Boost: Due to the longitudinal boost of the center-of-mass frame, quirk pairs produced with minimal transverse recoil are expected to travel predominantly along the beam axis into the forward or backward regions, matching the LHCb acceptance.
2. Geometric Topology: The oscillating string causes the quirk and anti-quirk to leave hits in the VELO that are:
   • Back-to-back: The azimuthal difference ($\Delta\phi$) between paired hits is close to $180^\circ$.
   • Co-planar: The hits lie in a single plane, with minimal variation in $\phi$ across different VELO modules.
   • Radially Aligned: The radial difference ($\Delta r$) between hits in facing modules is small (typically $< 5$ mm).
3. Simulation and Selection: The authors utilize a standalone code to simulate quirk trajectories through the precise VELO geometry, accounting for a single-hit spatial resolution of $\sim 10$ $\mu$m. The selection criteria require events to contain at least five reconstructed hit pairs in consecutive VELO stations satisfying the back-to-back and co-planar conditions.
4. Trigger Strategy: The analysis exploits the LHCb's fully software-based High Level Trigger (HLT). Unlike hardware triggers that might discard low-momentum or non-standard topologies, the software trigger allows for flexible, hit-level selections and the application of sophisticated offline Machine Learning (ML) algorithms to stored events, enabling the capture of these anomalous signatures.

Key Contributions

• Novel Search Channel: This paper introduces the first dedicated search strategy for quirks using the LHCb VELO, targeting the forward region where central detectors (ATLAS/CMS) have reduced sensitivity due to trigger requirements.
• Geometric Selection Criteria: The authors define robust geometric variables ($\Delta\phi$, $\Delta r$, and planarity) that remain stable across the quirk parameter space, distinguishing signal from background without relying on high-$p_T$ ISR.
• Background Suppression: The study demonstrates that the strict requirement for a sequence of back-to-back hits across multiple consecutive VELO stations effectively rejects combinatorial backgrounds and pile-up. Photon conversions are mitigated via material maps and vetoes, while ML-based offline tracking is proposed to further suppress residual backgrounds to negligible levels.
• Sensitivity Projections: The paper provides sensitivity projections for two scenarios: an integrated luminosity of $10$ fb$^{-1}$ (representing the 2026 dataset) and an ultimate dataset of $300$ fb$^{-1}$ (HL-LHC).

Results
The simulation results indicate that LHCb can probe parameter regions inaccessible to existing ATLAS and CMS searches.

• Exclusion Limits: The projected 95% confidence level (CL) exclusion contours show that LHCb can significantly constrain the $m_Q$ vs. $\Lambda$ plane, particularly in the region where $\Lambda \sim 1000$ eV and $m_Q$ ranges from 1 to 3 TeV.
• Efficiency: The detection efficiency is driven by the number of reconstructed hit pairs per event, which varies with $m_Q$ and $\Lambda$. The study finds that the geometric selection is robust, with the fraction of events satisfying the "at least 5 pairs" criterion remaining significant across the target parameter space.
• Background Hypothesis: Assuming the background can be reduced to negligible levels through the proposed geometric and ML-based cuts, the search offers a "background-free" discovery potential.

Significance and Claims
The paper claims that the unique combination of the LHCb VELO's forward coverage, high spatial resolution, and flexible software trigger provides a "powerful and distinct handle" to explore uncharted regions of the quirk parameter space. The authors emphasize that this approach offers high complementarity to central detector searches, potentially discovering quirks in regimes where current limits are weak.

The authors conclude that while the proposed search is robust for the 2026 run, future runs (Run 4 and Run 5) will benefit from enhanced timing capabilities of the 4D VELO upgrade. They advocate for the immediate development of dedicated trigger lines for these signatures. The paper maintains a modest tone regarding systematic uncertainties, noting that full validation requires detailed material interaction studies and the implementation of complete trajectory reconstruction algorithms, but asserts that the current geometric approach provides a viable and promising path toward discovery.